In some places where previously a notification bar appeared (like remembering log-in passwords or after installing an extension), SeaMonkey now shows a "doorhanger" below the location bar (bug 570004).

SeaMonkey now uses Omnijar which results in fewer installed files, less fragmentation and better startup time (bug 588067).

Sync (f.k.a. Weave) is now included directly in SeaMonkey (bug 576970).

Mac changes: PPC support was dropped, 64-bit support for 10.6 "Snow Leopard" was added (bug 599862).

The new DoNotTrack HTTP header can be configured in Preferences, Privacy & Security (bug 630270).